The Himalayas, the world’s tallest and most enormous mountain range, block the entry of freezing katabatic winds from the snowy Tibetan Plateau and northerly Central Asia. During winter, much of North India is, therefore, maintained warm or just slightly chilly or frigid; in summer, the same thermal dam keeps most of India hot. During the winter months, the weather conditions over India are mainly affected by the pressure distribution in Central and Western Asia. During the winter, a high-pressure centre forms in the area to the north of the Himalayas. These continental winds meet trade winds across northern India.

Mechanism of Weather in the Winter and Summer Season 

Surface Pressure and Winds

• During the winter months, the pressure distribution in Central and Western Asia has the greatest impact on the weather conditions in India
• A high-pressure centre forms in the Himalayan region to the north throughout the winter
• The centre of high pressure causes a low-level flow of air from the north to the Indian subcontinent, south of the mountain range
• The dry continental air mass arrives in India as a result of the surface winds from the high-pressure centre over Central Asia
• These continental winds collide with trade winds across north-eastern India
• On the other hand, the position of this contact zone is not stable
• It may migrate as far east as the middle Ganga valley on rare occasions, bringing dry north-western winds across the entire region of north-western and northern India, all the way up to the middle Ganga river

Jet Stream and Upper Air Circulation

• Jet streams can be seen up to 3 km above the earth’s surface in the lower troposphere
• Variations in atmospheric pressure closer to the earth’s surface have little bearing on the formation of higher air circulation
• Westerly winds continue to blow across Western and Central Asia at the height of 9-13 km from west to east
• At latitudes north of the Himalayas, jet streams sweep over the Asian continent, roughly parallel to the Tibetan highlands
• In these jet streams, the Tibetan highlands operate as a barrier
• As a result, jet streams split in two
• One of its branches reaches the Tibetan highlands to the north
• The southern branch of the wind sweeps eastward, south of the Himalayas
• In February, it has a mean position of 25°N with a pressure level of 200-300 Mb
• This southern branch of the jet stream is thought to significantly impact India’s winter weather

Western Cyclonic Disturbance and Tropical Cyclones 

• The westerly jet stream brings western cyclonic disturbances into India from the west and northwest during the winter months
• These disturbances begin over the Mediterranean Sea and are brought into India by the westerly jet stream
• The advent of these cyclones’ disturbances is usually preceded by an increase in the prevailing night temperature
• Tropical cyclones from across the Indian Ocean and the Bay of Bengal. The coasts of Tamil Nadu, Andhra Pradesh, and Odisha were affected by tropical storms with tremendous wind speeds and heavy rainfall
• Because of the intense wind speeds and copious rain accompanying them, most of these cyclones are pretty destructive

Mechanism of Weather in Summer Season

Surface Pressure and Winds

• The wind circulation over the subcontinent experiences a complete reversal when the sun moves northwards at both the lower and upper altitudes
• By the middle of July, the low-pressure belt near the surface [known as the Inter-Tropical Convergence Zone (ITCZ)] had shifted northwards, nearly parallel to the Himalayas between 20° and 25° north latitude
• The westerly jet stream had left the Indian subcontinent at this time
• The northward shift of the equatorial trough (ITCZ) and the withdrawal of the westerly jet stream from across the North Indian Plain are intertwined
• ITCZ is a low-pressure zone that attracts marine tropical air mass from the southern hemisphere, which rushes to the low-pressure area in a generally south-westerly direction after crossing the equator
• The southwest monsoon is the term used to describe this wet air circulation
• The troposphere is the only place where pressure and winds are formed
• In June, an easterly jet stream passes across the southern tip of the Peninsula
• It is restricted to 15oN latitudes in August and up to 22o N latitudes in September
• In the upper atmosphere, easterlies generally do not extend north of 30o N latitude

Easterly Jet Stream and Tropical Cyclones

• Tropical depressions are steered into India by the easterly jet stream
• These depressions influence the distribution of monsoon rainfall throughout the Indian subcontinent
• The areas of maximum rain in India are the tracks of these depressions
• The regularity with which these depressions pass across India and their direction and severity all affect the rainfall pattern during the southwest monsoon season

Conclusion

Weather is caused mostly by changes in air pressure, temperature, and moisture from one location to another. The significant temperature difference between polar and tropical air causes large scale atmospheric circulation cells and the jet stream. In India, during the winter, the northern region near the Himalayas experiences high pressure. As a result, winds from this area move south, where the pressure is lower. The northern section of the country has lower pressure in the summer.